Ultrawideband (UWB) technology has drawn considerable interest among the research and wireless communication communities due to its configurability and adaptability, which enables it to coexist with many concurrent services. YIG (yttrium-iron-garnet) based signal sources are known for their configurability, wideband tunability and ability to generate a spectrally pure signal at microwave frequencies. However, YIG based signal sources are costly, consume significant amounts of power, and are not particularly suited for fabrication by current integrated circuit (IC) technology. Nor are they immune from electromagnetic interference (EMI), vibration effects, microphonics, phase hit, and frequency modulation.
There are currently various mobile communication standards in use worldwide. Software-defined radio (SDR) enables the creation of multi-standard terminals, which may be used in various mobile communication systems by modifying their software. The coexistence of second and third generation wireless systems requires multi-mode, multi-band, and multi-standard mobile communication systems. These systems are expected to require a multi-octave-band signal source that replaces several narrow band voltage controlled oscillator (VCO) modules. In particular, it is generally desirable that these modules be replaced by a single UWB configurable spectrally pure signal source (i.e., a single voltage controlled oscillator).
Transceiver components such as VCOs, power dividers, amplifiers, and phase shifters are usually required to be capable of wideband performance to cover the frequency bands of various systems. The different standards operating in the frequency range of up to 6 GHz, and even higher frequencies with the introduction of UWB techniques, give rise to the need for wideband tunable sources. They also provide additional utility to a multi-standard radio frequency (RF) transceiver that combines several cellular and cordless phone standards as well as wireless LAN functionalities in one unit. This places more demand on the topologies and technologies used to implement reconfigurable multi-octave-band signal source operation with low-power and low phase noise characteristics. As the frequency band for wireless communications shifts higher, generation of a power efficient ultra low noise wideband and thermal stable compact signal at a low cost becomes more challenging due to the frequency limitations of the active devices. A high frequency signal can be generated either based on an oscillator operating at a fundamental frequency or a harmonic frequency.
Various approaches, such as frequency multipliers, switching between VCOs for separate bands, utilizing inter-modal multiple frequency, using switched resonators for band selection, are promising. These approaches result, however, in circuits having relatively large sizes that consume relatively large amounts of power, provide relatively poor noise performance and that are not cost effective. The drawback of a band-switching approach is power consumption and, where PIN diodes are employed, extra noise due to the switching spike generated from the PIN diodes.
More specifically, the use of signal frequency doublers or triplers in oscillators to multiply the frequency has disadvantages in that spurious signals are always present in the output. These spurious signals must be filtered out to avoid degrading receiver performance or causing interference with other radio services. In addition, the parts count increases greatly with doublers and triplers and the desired output frequencies must exactly match those multiples.
Phase noise is the noise that results from modulations in the oscillation or carrier frequency, of an oscillator and affects an oscillator's ability to be tuned precisely. In general, phase noise increases with frequency doubling and tripling. Furthermore, the phase noise performance of VCOs is becoming increasingly important with reduced communications channel spacing and more heavily loaded data transmissions. A wide tuning range and ultra low phase noise represent tradeoffs in the design of a VCO, impacting both the technology and the topology used. Multi-octave-band tunability and good phase noise performance have typically been assumed to be opposing requirements due to the problem of controlling the loop parameters and optimization of the time average loaded Q of the resonator over the band simultaneously.
There are a number of operational parameters that are of concern in oscillator operation depending on the oscillator's intended applications, but phase noise is an important figure of merit for measurement and instrumentation applications. For oscillators intended for fixed frequency operation it is relatively easy to optimize the parameters of particular concern. A problem is encountered, however, when the oscillator is tuned to operate over a wideband frequency range. For a varactor-tuned oscillator to continuously tune over a multi-octave-band, the tuning diode must typically exhibit a large change in capacitance in response to a small change in the tuning voltage. However, this enables the tuning diode's own capacitance to be easily modulated by the random electronic noise signals generated internally by various oscillator circuit elements, including the tuning diode itself. The tuning range of the VCOs directly influences the phase noise and there is a trade-off between the continuous multi-octave-band tunability of a VCO and the amount of phase noise generated by the varactor capacitance modulation. Low phase noise performance over the complete frequency range is a demanding requirement.
As mentioned above, some oscillators use PIN diodes. A disadvantage of PIN diodes in oscillators is that PIN diodes require significant DC current to obtain a low “ON” impedance, and when the PIN diodes are “OFF” they can create high levels of harmonically related spurious signals, losses and distortions. Furthermore, tank circuits associated with the PIN diodes reduce circuit Q, which reduce efficiency, and cause higher phase noise in the output circuit.
As also mentioned above, YIG resonator-based oscillators are well-known as wideband tunable voltage controlled oscillators, but at the cost of size, power and integrability in integrated circuit (IC) form. A YIG resonator is a magnetic insulator that resonates at a microwave frequency in the presence of the magnetic field. If the resonator is spherical, the frequency of resonance is related only to the strength of the magnetic field and not to the radius of the sphere. YIG resonators are usually made of either single-crystal yttrium iron garnet or gallium-substituted yttrium iron garnet. In a YIG oscillator, a YIG sphere is used as a reactive component, and it is placed in a magnetic field to set its resonant frequency. For a tunable YIG oscillator, the YIG sphere is placed in the air gap of an electromagnet, and the current applied to the windings is varied as desired in order to obtain the desired frequency of oscillation. Consequently, YIG oscillators are usually large, heavy and consume relatively large amounts of power, and therefore are not typically suited to fabrication by current IC technology. In addition, YIG based oscillators are usually sensitive to vibration, microphonics, phase hits, and frequency-agility.
In that regard, a transceiver module may presently be implemented on a single IC chip, except for the YIG stage resonators. Therefore, to reduce the transceiver cost on a single IC chip, it is desirable to eliminate the YIG resonator. One way to eliminate the YIG resonator is to use a planar resonator. But a planar resonator suffers from a relatively low Q (quality factor) and is therefore susceptible to phase noise.
To solve the frequency-agility issue, radio manufacturers that serve the point-to-point and point-to-multi-point markets generally prefer to use an oscillator that is configurable, wideband tunable and provides relatively low phase noise. This avoids a YIG based signal source so as to provide higher transmission speeds at reduced cost and power. Such oscillators may then be advantageously used for present and later generation communication systems.
Unlike conventional signal sources (i.e., conventional oscillators/VCOs), a YIG based oscillator's quality factor (Q) increases with frequency, particularly at millimeter wave frequencies. A YIG based synthesizer provides low noise performance and is broadband tunable when compared to the standard signal sources. However, a YIG based oscillator requires a significant amount of power (>24V, 100 mA). This results in generation of excessive heat, which may harm the other electronic components in the transceiver modules. In addition, YIG based oscillators are usually prone to vibration, lighting, electromagnetic interference (EMI), microphonics, phase hits, and frequency modulation, all of which have a detrimental effect in designing modern communication systems. The above effects may cause interruptions in the carrier signal and affect the radio's bit-error-rate (BER).
In the past, YIG oscillators have employed either a FET or a bipolar transistor as the active device coupled to the YIG resonator. FETs can generally operate at higher frequencies than bipolar transistors, but bipolar transistors have significantly better 1/f noise characteristics. No single broadband device has been available that can be tuned to frequencies with both the bipolar and FET microwave frequency ranges. Attempts have been made to increase the high frequency limit of bipolar transistor-based YIG oscillators by increasing the high frequency limit of the transistors, but these transistors have also tended to have higher minimum frequencies of operation. Further, YIG oscillator circuits are usually designed to either operate at low frequencies or high frequencies. It is usually challenging and difficult to design a single circuit, which may be tunable as a broadband source.
In view of the limitations of known YIG resonators for integrability and power-effective operation, there is a need for compact size oscillators that support multi-octave-band tunability and that are amenable to integration in chip form. In addition, there is a need for a broadband tuned oscillator packaged as a single device that can be used in place of the YIG oscillator.